Finish exercise 11
 * Exercise 11: (2) Create a private inner class that implements a public interface. Write
a method that returns a reference to an instance of the private inner class, upcast to the
interface. Show that the inner class is completely hidden by trying to downcast to it.
* This do not need try, but update a comment in exercise 4, within a class, inner private class can use downcast, but no downcast forinnerclass inside a methods.